System and method for measuring stress during processing of an optical fiber

ABSTRACT

A system and method for measuring stress exerted on an optical fiber including providing an optical fiber that includes a fiber optic sensor, and exposing the optical fiber and the fiber optic sensor to various stresses associated with the process by moving the optical fiber and the fiber optic sensor through the process to be measured. The system and method further includes transmitting a source light through the optical fiber as the optical fiber and the fiber optic sensor are exposed to various stresses, receiving a return light signal from the fiber optic sensor as the optical fiber and the fiber optic sensor are exposed to various stresses, and comparing the source light signal to the return light signal for determining the stress exerted on the optical fiber.

This application is a divisional application of U.S. Ser. No. 09/829,783filed on Apr. 10, 2001 U.S. Pat. No. 6,366,711 which is a divisionalapplication of and claims priority to and the benefit of U.S. Ser. No.09/407,579, filed Sep. 28, 1999, U.S. Pat. No. 6,314,214.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for measuring stress exertedon an optical fiber, and in particular to a method for measuringstresses exerted on an optical fiber during manufacturing of the fiberor of a cabled fiber.

2. Technical Background

Numerous forms of fiber optic sensors have been developed to monitorparameters in various systems and processes, including the Fabry-PerotInterferometer, the Bragg Grating, the Mach-Zehnder Interferometer, andthe Michelson Interferometer, to name but a few. These fiber opticsensors are used in a wide variety of applications, including use asstrain gauges, dynamic pressure sensors, bearing condition sensors,non-contact proximity sensors, and temperature sensors. In each of theseapplications, the fiber optic sensor is fixedly attached to the systemto be monitored, and usually is encased within a housing or rigidstructure that is fixedly attached to the system to communicateparameter changes in the system to the fiber optic sensor.

As strain gauges, fiber optic sensors have been used to monitor dynamicstrain. In such applications, the fiber optic sensor is imbedded withina material that is attached to a component of a structure such that thestrain within the component may be monitored. Applications of fiberoptic strain gauges have typically included civil structures such asdams, buildings, and bridges.

As dynamic pressure sensors, fiber optic sensors have been used in avariety of applications including the monitoring of performance ofinternal combustion engines, as well as monitoring the performance ofcompressors and pumps. When used to monitor the performance of aninternal combustion engine, the fiber optic sensor is typically placedwithin a housing mated with a cylinder of the engine. The housingtypically has a metal diaphragm that is attached to one end of the fiberoptic sensor. Pressures exerted on the diaphragm are transferred to thefiber optic sensor, thereby changing the overall length of the sensorand allowing measurement of continuous real-time in cylinder pressurespermitting improved engine control, providing preventive maintenancedata, and predictive emissions monitoring. When used to monitor theperformance of compressors and pumps, the fiber optic sensor is imbeddedwithin an aluminum alloy rod, or similar metal, by an encasing process.The aluminum rod encasing the fiber optic sensor is then placed within ametal housing having a diaphragm similar to that described above inrelation to engine monitoring. By placing the diaphragm in contact withthe fluid being transferred by the compressor and/or pump, measurementsof cavitation, flow instability, and surge detection are possible,thereby reducing the risk of catastrophic mechanical failure.

As bearing condition sensors, fiber optic sensors are used to monitorthe condition of bearing or rotor imbalance. Typically, the fiber opticsensor is encased within a housing that includes a deformable diaphragm.The fiber optic sensor is in contact with the diaphragm which is, inturn, in contact with the outer race of a bearing, thereby allowing forthe transfer of any vibrations between the associated bearings and theouter race to the fiber optic sensor.

In non-contact proximity sensors, fiber optic sensors are used tomeasure shaft vibration, rotor thrust position, shaft rotational speed,as well as rotor imbalance and misalignment. In these applications, thefiber optic sensor is encased within a steel rod having a magnetattached to an end thereof. The steel rod encasing the optical fiber andthe magnet are positioned within a stationary housing. The housing isthen located such that the magnet is in close proximity to the rotatingshaft to be monitored. Imbalances in the shaft cause the magnet to movewhich motion is transferred to the optical sensor for monitoring of theposition or condition of the shaft.

As temperature sensors, fiber optic sensors are typically inserted intoareas desired to be monitored, or imbedded into cast parts, therebyallowing the direct measurement of temperatures therein.

Typically, fiber optic sensors have been used to monitor systems thatallow for stationary or fixed placement of the sensor within the system.The construction of these sensors have made it difficult if notimpossible to monitor processes, systems, or machines that require theoptical fiber and the associated fiber optic sensor to be movedthroughout the system being monitored. Further, these systems typicallyrequire the fiber optic sensor to be cast within a part or structure tobe monitored, or placed within a housing that is attached directly tothe system to be monitored, thereby adding to the size and costassociated with the monitor system.

The manufacturing procedures and processing of optical fibers and fiberoptic cables are numerous and varied. Many of these processes includeplacing a stress on the optical fiber or fibers being processed. Thesestresses when applied over time, however short, result in sub-criticalgrowth of the pre-existing flaws located within the optical fibers,thereby decreasing the overall strength of the optical fiber. In certainapplications, it is important that the optical fiber, or bundle offibers, has sufficient strength to withstand loads place thereon withoutdamaging the optical fiber or overall fiber optic cable. As a result,reliability models are created to estimate the strength of the fiber andthe associated fiber optic cables after the processing andmanufacturing. Reliability models for optical fibers are based on threethings: the size distribution of flaws or cracks within the fiber;fatigue crack growth parameters; and the stress-time profile which thefiber experiences during processing. High-stress processing events mayresult in degradation of the fiber strength. Until now, directmeasurements of the stresses exerted on an optical fiber duringhigh-speed processing has not been possible, and, as a result, thestress-time profile of optical fiber has been an assumed quantity.

The ability to collect real-time measurements of the stresses exerted onan optical fiber during processing and cable manufacturing would bevaluable for reliability analysis and modeling, process and equipmentdesign, trouble-shooting of manufacturing lines, as well as fiber andcable installation.

SUMMARY OF THE INVENTION

One aspect of the present invention is to provide a method and systemfor measuring stress exerted on an optical fiber including providing anoptical fiber that includes a fiber optic sensor, and exposing theoptical fiber and the fiber optic sensor to various stresses associatedwith a process by moving the optical fiber and the fiber optic sensorthrough the process to be measured. The method and system furtherinclude transmitting a source light signal through the optical fiber asthe optical fiber and the fiber optic sensor are exposed to the variousstresses, receiving a return light signal from the fiber optic sensor asthe optical fiber and the fiber optic sensor are exposed to the variousstresses, and comparing the source light signal to the return lightsignal for determining the stresses exerted on the optical fiber.

Another aspect of the present invention is to provide a method and asystem for measuring stress exerted on an optical fiber includingproviding a light source emitting a light having a predeterminedfrequency, providing a first photo detector coupled to the light sourcethat produces a first electrical signal proportional to the light,providing a first adjustable amplifier coupled to the first photodetector which amplifies a first electrical signal therefrom, andproviding the optical sensor coupled to the light source by the opticalfiber to transmit at least a portion of the light to the optical sensor.The method and system further includes providing a second photo detectorcoupled to the optical sensor that detects at least a portion of thelight reflected from the optical sensor, and produces a secondelectrical signal proportional thereto, providing a second adjustableamplifier coupled to the second photo detector that amplifies the secondelectrical signal therefrom, providing a comparator coupled to thesecond photo detector that compares the first and second electricalsignals to provide a signal representative of the relationship betweenthe first and second signals. In one embodiment, a microcontroller iscoupled to the comparator and generates a plurality of trigger signalsat a fixed frequency, each initiating a modulated cycle and further thatgenerates a control signal in response to detecting a predeterminedtransition between the first and second output voltages. The method andsystem further includes providing a modulator coupled to the lightsource and the microcontroller to modulate the light source in aperiodic manner to provide pulses in response to receiving a triggersignal from the microcontroller. A counter is coupled to themicrocontroller and begins counting the periodic pulses in response toreceiving a trigger signal, and ends counting in response to receiving acontrol signal from the microcontroller to generate a count value. Theoptical fiber and the fiber optic sensor is exposed to various stressesassociated with a process by moving the optical fiber and the fiberoptic sensor through the process to be measured, and the microcontrollercomputes the stresses exerted on the optical fiber in response toreceiving the count value for each modulation cycle.

Yet another aspect of the present invention is to provide a method fordetermining the amount of stress exerted on an optical fiber thatutilizes a source of light signals, a detector for detecting a returnlight signal, a comparator for comparing the emitted light signal to thereturn light signal, and a microcontroller for calculating the stressexerted on the optical fiber. The system includes providing an opticalfiber, with a fiber optic sensor. The optical fiber and the fiber opticsensor is exposed to various stresses associated with a process bymoving the optical fiber and the associated fiber optic sensors throughthe process to be measured, while monitoring the detected light signalsfor calculating the stress exerted on the optical fiber during theprocess.

Additional features and advantages of the invention will be set forth inthe detailed description which follows and will be apparent to thoseskilled in the art from the description or recognized by practicing theinvention as described in the description which follows together withthe claims and appended drawings.

It is to be understood that the foregoing description is exemplary ofthe invention only and is intended to provide an overview for theunderstanding of the nature and character of the invention as it isdefined by the claims. The accompanying drawings are included to providea further understanding of the invention and are incorporated andconstitute part of this specification. The drawings illustrate variousfeatures and embodiments of the invention which, together with theirdescription serve to explain the principals and operation of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a manufacturing process for an opticalfiber employing the method and sensing system of the present invention;

FIG. 2 is multiple cross-sectional views showing an optical fiber andthe steps to construct a Fabry-Perot Interferometer;

FIG. 3 is an electrical circuit diagram in block and schematic form ofthe system of the present invention;

FIG. 4 is a schematic view of a coloring die and a correspondingstress-time graph obtained from the system shown in FIG. 3; and

FIG. 5 is a side view of a frictional trigger generator employed in thesystem of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, a sensing system 10 embodying the present invention isemployed in the processing of an optical fiber 12 having a fiber opticsensor 14 imbedded therein. An electrical circuit 16 is connectedthrough fiber coupler 56 to optical fiber 12 for monitoring a process,system, or machine 18 treating the optical fiber 12.

The optical fiber 12 can be a single-mode optical fiber that includes acore 20 (FIG. 2), a cladding 22, and a polymeric outer coating 24. Asingle-mode optical fiber, as designated herein, is optical fiber thatpropagates only two mutually orthogonal polarization modes of the HE11mode of light, at a chosen signal wavelength. Further, while a CorningSMF28™ fiber has proven effective, other types and classifications ofoptical fibers may be used. The optical fiber 12 is preferably providedon a conically-shaped spool 25 (FIG. 1) to allow free, unrestrictedunspooling and delivery of the optical fiber 12 into the process orsystem 18 to be monitored, however, optical fiber 12 can be provided inany form that allows free, unrestricted delivery.

The most preferred fiber optic sensor 14 utilized in the presentinvention is a Fabry-Perot Interferometer 26 (FIG. 2). While the mostpreferred fiber optic sensor 14 is the Fabry-Perot Interferometer 26,the described signal processing system may also be applied to othersensors, such as Michelson and Mach-Zehnder Interferometer sensors. Inaddition, the optical power can be provided to sensors monitored intransmission as well as in reflection such as in a fiber Bragg gratings,and others as known in the art. The sensing system 10 employs themeasurement of the relative phase shift within a Fabry-Perotinterferometer 26. It is assumed that the measurement of interest, thestress exerted on the optical fiber 12 and the embedded Fabry-PerotInterferometer 26, affects the optical phase shift in the Fabry-PerotInterferometer 26 and that the value of the strain is related to thephase shift through a calibration factor. Suitable mathematicalcomputations associated with calculating the phase shift and the strainsassociated therewith are disclosed, for example, in U.S. Pat. No.5,557,406.

The Fabry-Perot Interferometer 26 as used in the present system 10 isconstructed or disclosed in FIG. 2 by first removing the polymeric outercoating 24 from about a portion of optical fiber 12, as shown in step 1.The optical fiber 12 is then cut forming a first cleaved end 28 and asecond cleaved end 29, using, for example, a Fujikura model 30SF splicer25 as shown in step 2. A first dielectric mirror 30 of titanium dioxideor similar compound is then fabricated by vacuum deposition, or asimilar method, on cleaved end 28, as shown in step 3. Second cleavedend 29 is then spliced onto first dielectric mirror 30 by way ofelectric arc fusion, or a similar method, as shown in step 4. Opticalfiber 12 is then re-cut a distance L from first dielectric mirror 30,thereby forming a third cleaved end 32 and a fourth cleaved end 33, asshown in step 5. A second dielectric mirror 34 of titanium dioxide orsimilar compound is formed on third cleaved end 32, shown in step 6.Fourth cleaved end 33 is then spliced onto second dielectric mirror 34,thereby reforming optical fiber 12 into a continuous length of fiber andforming the Fabry-Perot Interferometer 26 therein, as shown in step 7.Distance L can be formed so as to be between about 0.5 millimeters (mm)and 10 meters. Preferably, distance L is between 5 and 25 millimeters.Most preferably, distance L is approximately 12 millimeters. Opticalfiber 12, including interferometer 26 is then re-coated with outercoating 24 along the area from which outer coating 24 was previouslyremoved, as shown in step 8. The adhesion between the dielectric mirrors30 and 34 and the optical fiber 12 is sufficient to withstand thestresses exerted on the optical fiber 12 and the associated fiber opticsensor 14 as the optical fiber 12 travels through the process or system18.

As an alternative to dielectric mirrors, Bragg gratings can be used toform the Fabry-Perot cavity. One would write the Bragg gratings at theappropriate spacing, L. In order to achieve broadband reflectionencompassing the wavelength of a light signal, for example, from alaser, the gratings would be chirped.

The electrical circuit 16 (FIG. 1) includes a semiconductor laser diodeor light emitting device 36 coupled to an end of fiber 12 and whichemits a source light signal 44 at a predetermined frequency or set offrequencies. A first photo detector 38 is coupled to source 36 toconvert at least a portion of the source light signal 44 into anelectrical signal. A second photo detector 40 is coupled through coupler56 to optical fiber 12 to detect a return light signal 46 and provide asecond electrical signal representative thereof. A comparator 42 iscoupled to detectors 38 and 40 to compare the first and secondelectrical signals. Microcontroller 48 is coupled to comparator 42 andanalyzes the signal therefrom to calculate the stress being exerted uponthe optical fiber 12 and the associated fiber optic sensor 14. Circuit16 has an output terminal 50 for providing a signal to a dataacquisition system 52 such as an oscilloscope or an appropriateprocessor that allows the operator to monitor the output of the system,in real time. A detailed description of the circuit 16 is now presentedwith reference to FIG. 3.

In FIG. 3, a system clock 64 generates and sends a clock pulse to afunction generator 54 and the microcontroller 48. The function generator54 produces an appropriate current periodic waveform in response to theclock pulse for modulating the source light signal 44 from the laserdiode 36 through a modulation cycle. The signal generated by thefunction generation 54 is converted to a digital signal by a analog todigital converter 55 and combined with the signal from themicrocontroller 48 by a variable gain amplifier 57. The gain ofamplifier 57 is controlled by the signal applied to a gain control inputfrom microcontroller 48. The resultant output signal from the amplifier57 is then coupled to a laser driver amplifier 59, which in turn,provides the source signal to laser diode 36. The first photo detector38 receives a portion of the laser output power from the laser diode 36and generates an electrical signal proportional to the optical powerreceived by first photo detector 38. An optical isolator (not shown) mayoptionally be provided to prevent reflected light from destabilizing thelaser output. The light propagated in the optical fiber 12 is then splitby a fiber coupler 56 to provide optical power to the fiber optic sensor14 located upstream of the processing area 18.

A portion of the source light signal 44 is reflected by the fiber opticsensor 14 as a function of stress on the optical fiber 12 creating areturn light signal 46 that is routed through the fiber coupler 56 tothe second photo detector 40, which converts the return light signal 46into an electrical signal proportional to the optical energy reflectedfrom the sensor 14.

The electrical signal from the second photo detector 40 serves as theinput to a second variable gain operational amplifier 59 which has oneor more gain stages controlled by a signal applied to an input terminal49 from microcontroller 48. The output signal from amplifier 59 isprovided as one input to comparator 42. The signal from first photodetector 38 is routed to an operational amplifier 60 which provides anoutput signal that serves as the other input to the comparator 48. Thegain of amplifier 59 is adjusted so that the two comparator inputsignals cross one or more times during a modulation cycle, so that eachtime a crossing occurs, the output of the comparator 48 changes from alogic “low” to logic “high” state and vice versa. At the time amodulation cycle begins, a first control signal is sent by themicrocontroller 48 to a counter 62 to initiate the counting of clockpulses generated internally or received from the system clock generator64 via conductor 63. Starting at a preprogrammed time after thebeginning of each modulation cycle, the microcontroller 48 monitors theoutput signal from the comparator 42 to determine when a selectedtransition occurs in the output signal of the comparator 42. When such achange is identified, the microcontroller 48 sends a second controlsignal to the counter 62 to stop counting. The accumulated count duringthe time from the initiation of the cycle to the end of the cycle isthen provided to the microcontroller/microprocessor 48, where it isfurther processed. The microprocessor 48 may process the accumulatedcount by baseline subtraction to remove common or DC components and thenmultiplication by a calibration factor as described in the aboveidentified '406 patent to produce a digital output whose value equalsthat of the stress exerted on the optical fiber 12 at any given time.

As the optical fiber 12 is run through a process or system 18 whichplaces stresses upon optical fiber 12 and the fiber optic sensor 14,physical stress exerted on optical fiber 12 results in a change oflength of optical fiber 12 including that section which makes up thefiber optic sensor 14. As discussed above, this change in length resultsin a relative phase shift of the light signals 44 and 46 within thefiber optic sensor 14 which is converted into a strain reading which, inturn, is then converted into real-time stress readings, thereby allowingfor accurate determination of the stress-time profile exerted on theoptical fiber.

Sensing system 10 can be used to effectively monitor those processes andsystems associated with high speed processing of optical fiber and fiberoptic cable manufacturing, as well as the installation of each. Theseapplications of the present sensing system 10 include measuring theunload rate during proof testing of an optical fiber, thereby ensuringthat the optical fiber meets industry strength standards andrequirements. In addition, sensing system 10 can be used in determiningthe stress transfer of varying belt and capstan designs, as well asdeveloping stress profiles that occur during coloring, ribboning andstranding processes. Further, sensing system 10 can be used totroubleshoot processes associated with manufacturing fiber optic cablessuch as finding worn bearings as well as high stress points within theprocess. The processes as listed above are in no way intended to belimiting on the applicability of the present invention and are providedas merely exemplary applications.

By way of one example, as seen in FIG. 4, the present invention could beused to determine the amount of stress exerted on optical fiber 12 whenprocessed through a coloring die 66. As known in the art, coloring diesare used to apply a colorant 69, usually provided in the form of a UVuncured acrylate, to an optical fiber. As shown in the correspondinggraph of FIG. 4, the coloring die 66 exerts a stress on optical fiber 12due to a frictional force between optical fiber 12 and coloring die 66.As the optical fiber 12 travels in a direction 68 through coloring die66, the frictional forces acting upon the integral Fabry-PerotInterferometer 26 results in a change in length thereto, therebyallowing for the calculation of the stress exerted on the optical fiber12 as described above.

The generated stress versus time profile as seen in FIG. 4 and allowsfor accurate determination of the stress exerted on the optical fiber 12when it passes through coloring die 66. The stress-time profile can thenbe used for purposes such as troubleshooting the associated opticalfiber production line, optimizing die tension, calculating the stresshistory of the optical fiber to develop accurate optical fiber strengthreliability models, as well as numerous applications.

Another aspect of the present sensor system 10 is to provide a frictiontrigger 68 (FIGS. 1 and 5) located at a fixed position relative to themovement of the optical fiber 12 as optical fiber 12 travels through theprocess or machine 18 to be monitored. Friction trigger 68 includes afirst body 70 having an inner face 71, and an opposed second body 72having an inner face 73. A cloth cover 74 is secured to inner face 71 offirst body 70 and to the inner face 73 of the second body 72. Acompression spring 76 holds the cloth covers 74 secured to the firstbody 72 and second body 74 in close contact. In operation, frictionaltrigger 68 is positioned about optical fiber 12 (extending into and outof the place of drawing FIG. 5) so as to provide a slight frictionalforce therebetween. When the fiber optic sensor 14 passes throughfriction trigger 68 a slight increase in the stress exerted on theoptical fiber 12 occurs, thereby providing the user with a point in timefrom which to begin reading the stress forces as encountered by fiberoptic sensor 14 as it travels through process or machine 18. By knowingthe location of the friction trigger, the speed which the optical fiber12 travels through the process or machine 18, and the distance betweenthe components within the process or machine 18 that will exert a stresson the fiber optic sensor 14, the operator can determine the stressexerted on the optical fiber 12 by each component of the process ormachine 18. The description of the frictional trigger 68 as providedabove is in no way intended to limit the configuration of the triggerdevice or its function in the process.

It will be apparent to those skilled in the art that variousmodifications and adaptations can be made to the present inventionwithout departing from the spirit and scope of this invention. Thus, itis intended that the present invention, provided they come within thescope of the appended claims and their equivalents.

The invention claimed is:
 1. A system for measuring stress exerted on anoptical fiber, comprising: an optical fiber that includes a fiber opticsensor; a light source emitting a source light signal into the opticalfiber, thereby generating a return light signal from the fiber opticsensor; and an apparatus capable of receiving the return light signalfrom the fiber optic sensor, comparing the source light signal to thereturn light signal, and determining the stress exerted on the opticalfiber; wherein when the optical fiber and the fiber optic sensor isexposed to various stresses associated with a process by moving theoptical fiber and the fiber optic sensor through the process to bemeasured, the apparatus determines the stress exerted on the opticalfiber.
 2. The system of claim 1, wherein the optical sensor includes anoptical fiber interferometer.
 3. The method described in claim 2,wherein the optical fiber interferometer is a Fabry-PerotInterferometer.
 4. The method described in claim 1, wherein the fiberoptic sensor is a fiber Bragg grating.